The Rebel XSi/450D image sensor is 1.6 times smaller than a traditional 35mm film frame. It is important to know the sensor size because it not only determines the size of the image but also affects the angle of view of the lenses you use. A lens's angle of view is how much of the scene, side to side and top to bottom, that the lens includes in the image. For example, a 15mm Fisheye lens has a 180-degree angle of view. By contrast, a 200mm lens has a scant 12-degree angle of view.
Further, the angle of view for all lenses you use on the XSi/450D is reduced by a factor of 1.6 times at any given focal length, producing an image that is equal to that of a lens with 1.6 times the focal length. That means that a 100mm lens on a 35mm film camera becomes the equivalent of a 160mm on
In This Chapter
Understanding the focal length multiplication factor
Zoom versus prime lenses
Canon lens terminology
Using wide-angle lenses
Using telephoto lenses
Using normal lenses
Using macro lenses
Using tilt-and-shift lenses
Using Image-Stabilized lenses
the XSi/450D. Likewise, a 50mm normal lens becomes the equivalent of an 80mm lens, which is equivalent to a short tele-photo lens on a full 35mm-frame size.
Tip The EF-S lenses are usable only on the cropped-frame cameras, including the XSi/450D, due to a redesigned rear element that protrudes back into the camera body.
This focal length multiplication factor works to your advantage with a telephoto lens because it effectively increases the lens's focal length (although technically the focal length doesn't change). And because tele-photo lenses tend to be more expensive than other lenses, you can buy a shorter and less expensive telephoto lens and get 1.6 times more magnification at no extra cost.
The focal length multiplication factor works to your disadvantage with a wide-angle lens because the sensor sees less of the scene when the focal length is magnified by 1.6x. But, because wide-angle lenses tend to be less expensive than telephoto lenses, you can buy an ultrawide 14mm lens to get the equivalent of an angle of view of 22mm.
Because telephoto lenses provide a shallow depth of field, it seems reasonable to assume that the conversion factor would produce the same depth-of-field results on the XSi/450D that a longer lens gives. That isn't the case, however. Although an 85mm lens on a full 35mm-frame camera is equivalent to a 136mm lens on the XSi/450D, the depth of field on the XSi/450D matches the 85mm lens, not the 136mm lens.
This depth-of-field principle holds true for enlargements. The depth of field in the print is shallower for the longer lens on a full-frame camera than it is for the XSi/450D.
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